1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof, and more particularly, to an image forming apparatus and a control method thereof improving an image quality.
2. Description of the Related Art
An image forming apparatus forms an image corresponding to printing data on a printing medium, and includes an electric copier, a printer, a scanner, a facsimile, a multifunction device integrating a part or all of functions thereof, etc.
As shown in FIG. 1, a conventional image forming apparatus 1 includes an image carrying body 13, a charging roller 11 charging a surface of the image carrying body 13, a light exposing unit 16 exposing the surface of the charged image carrying body 13 to form an electrostatic latent image corresponding to printing data, a developing roller 12 applying a developer to the electrostatic latent image of the surface of the image carrying body 13 to form a visible image, and a transferring roller 14 transferring the developer on a printing medium P.
However, if a gray image is outputted by using the conventional image forming apparatus 1 of FIG. 1, then an image having a stripe in a middle part thereof may be outputted as illustrated in FIG. 2A. FIG. 2B simplifies the image pattern of FIG. 2A, and shows that the stripe shown in FIG. 2A is visible to the naked eye in case of an image pattern configured with a black image area b having a high density in a front end of the printing medium P, and a gray image area g having a slightly low density next to the black image area b.
As shown in FIGS. 2A and 2B, it shows that an unexpected stripe j occurs to a portion distanced from the deep black image area b by approximately 75 mm apart.
FIGS. 3A to 3C are actual measuring graphs respectively measuring variations according to time of a surface electric potential of the image carrying body 13, a feedback voltage of the transferring roller 14 and a transferring voltage of the transferring roller 14 when an image of FIG. 2B is printed by using the conventional image forming apparatus 1. The surface electric potential of the image carrying body 13 is measured by means of a non-contracting sensor 15 sensing a surface electric potential.
The letter c in FIG. 3A shows that the surface electric potential varies from approximately −700V to −150V as a surface of the image carrying body 13 corresponding to the width w of the deep black image area b of the front end of the printing medium is totally exposed by means of the light exposing unit 16. The letter k in FIG. 3A highlights the surface electric potential of the image carrying body 13 varies from approximately −700V to −350V by means of the light exposing unit 16 to correspond to the gray image area g which follows the deep black image area b.
In FIG. 3B, the feedback voltage of the transferring roller 14 is a voltage feedback according to time, which is measured by applying a voltage for sensing to the transferring roller 14. If the image carrying body 13 and the transferring roller 14 are respectively regarded as resistors and the voltage for sensing is applied thereto as power, a kind of virtual closed circuit is configured. The feedback voltage is a voltage converted from a current flowing through the virtual closed circuit. The letter e in FIG. 3B highlights the moment when the printing medium P enters a transferring nip N between the image carrying body 13 and the transferring roller 14. The drop in feedback voltage highlighted by e occurs because the printing medium P may be regarded as a resistor newly added to the virtual closed circuit, and accordingly, the feedback voltage decreases.
As shown in FIG. 3C, the transferring voltage of the transferring roller 14 increases coincidentally at a point in time when the printing medium P enters a transferring nip N between the image carrying body 13 and the transferring roller 14.
The letter f in FIG. 3B highlights that the feedback voltage abruptly decreases when a surface of the image carrying body 13 corresponding to a portion c in FIG. 3A enters the transferring nip N.
Also, the letter d in FIG. 3A shows that the surface electric potential of the image carrying body 13 exposed to print the gray image area g in FIG. 2(B) is overshot, and the absolute value thereof becomes smaller than a circumference. This means that if there is a potential difference rapid change section m in which a sudden potential difference is generated to the surface of the image carrying body 13 at about time t1, an effect thereof still exists although the potential difference rapid change section m passes through the charging unit 11.
More specifically, although the image carrying body 13 makes one revolution so that the potential difference rapid change section m can be exposed again by means of the light exposing unit 16 to print the gray image area g, a peak value thereof reaches a surface electric potential larger (the absolute value thereof is smaller) than a surface electric potential of the circumference, −350V as represented as d in FIG. 3A. Accordingly, since a developer charged with a negative charge is concentrated to a surface of the image carrying body 13 having a relatively smaller electric potential than the circumference (a part corresponding to t3 in FIG. 3A), the stripe j becomes visible to the naked eye as shown in FIGS. 2A and 2B, that is, an image ghost appears.